The invention relates generally to x-ray imaging and more particularly to an x-ray system for producing three-dimensional images.
Substantial development has been made in the field of x-ray imaging since the discovery of the penetrating capability of x-ray radiation. In many applications, radiography is still utilized to produce a simple two-dimensional projected image, but developments have been made to add modalities to this framework. Although developed in the 1930""s, mathematical algorithms for tomography (i.e., the reconstruction of a three-dimensional image of an object from a set of cross-sectional two-dimensional detections) have only recently been exploited, with the evolution of computer-aided tomography (CAT) technology.
In the medical field a CAT system is used for detecting images representing internal parts of a patient""s body, such as a heart or stomach, for subsequent diagnosis and treatment. A typical medical CAT system synchronously rotates a radiation source and a corresponding singular large array detector to xe2x80x9cscanxe2x80x9d a set of radiographs and then compute slices of planar cross-sectional views of the interior of a human body utilizing reconstruction algorithms.
In the electronics part fabrication field, a CAT system may be used during a quality control phase to monitor the state of soldered joints for electrical components, such as printed circuit boards (PCBs). Without requiring physical, visual or electrical access, defects such as shorts, opens, insufficient/excess solder, misaligned and missing components, and reversed polarized capacitors, can be detected. Other products commonly subjected to x-ray inspection include cellular and wireless phones, notebook computers, routers, switches, and PC motherboards. A typical electronic CAT system includes a fixed x-ray source for projecting imaging radiation onto a movable object. A fixed continuous detector array located on a side of the object opposite to the x-ray source captures the sampled radiation that has passed through the object. In a case where the area of the sampled object to be imaged is larger than a field of view of the imaging system, the object and/or the source must be moved in order to obtain multiple views. A disadvantage with this approach is that there is a time delay associated with the repetitive start-and-stop motion, since data relating to the sampled radiation cannot be continuously taken.
In the field of tomography, an important process parameter in collecting data for assembling three-dimensional information is to obtain a wide range of angular projections of the radiation that has passed through the sampled object. The tomographic angle is defined for each arbitrarily small region of the sampled object as the angular range of the projected views that have been collected through this region. For example, two rays through a point inclined oppositely 30 degrees from the normal to the sampled object would create a data set having a tomographic angle of 60 degrees. Accordingly, a large number of two-dimensional views captured by an x-ray detector at different angular projections is required in order to build a sufficient data set. Mathematical algorithms reconstruct the three-dimensional image by computationally combining the two-dimensional views captured at the various angles. This computational combination can be either tomographic or tomosynthetic. In tomographic reconstruction, non-linear reconstruction algorithms converge a hypothetical three-dimensional description to the available data set. In tomosynthetic reconstruction, reasonably simple arithmetic and linear operations calculate a three-dimensional description from the available data set.
One conventional x-ray detection technique utilizes photosensitive film for capturing an image. However, the drawbacks of utilizing film include the use of chemicals for film development and the requirement of a time-consuming development process. Recent advances have eliminated the need of film. In one available system, a scintillator converts x-ray radiation that has propagated through the sampled object of interest into visible light, and a charged coupled device (CCD) converts the light into electrical signals for processing in the digital domain. Other filmless systems employ complementary metal oxide semiconductor (CMOS) pixel sensors.
Notwithstanding the advances made in x-ray detection techniques, an array of sensor elements for capturing a contiguous image is commonly used. Several disadvantages and problems are associated with a continuous array of sensor elements. First, if any detecting element (e.g., one pixel sensor) of the array become defective, replacement of the entire array may be required. Second, if the image captured by the continuous array includes regions of the sampled object not required for diagnosis, the data acquisition rate is unnecessarily extended. Third, since it is critical in collecting images for use in computing three dimensional information to obtain the largest possible tomographic angle in the data set, the requirement of a single contiguous detector can either limit the tomographic angle for a given active area of detector or increase the cost unnecessarily.
Consequently, what is needed is an x-ray imaging system having a detecting arrangement that allows for reliability, efficiency, and manufacturing and operational cost savings.
The invention is an x-ray imaging system that utilizes multiple detecting modules distributed in a sparse configuration for detecting sub-image data sets with a large tomographic angle of regions of a three-dimensional object. The x-ray imaging system comprises: (1) an x-ray source for projecting pulses of imaging radiation onto a sampled object, (2) a support member on which the sampled object is placed, and (3) a detector assembly having multiple detecting modules sparsely distributed for detecting imaging radiation that has passed through the object. A sparse configuration is herein defined as an arrangement of detecting modules in which each detecting module is spaced apart from an adjacent detecting module by an intermediate distance. In one embodiment, the intermediate distance is greater than the pixel spacing within the module. A projected pulse of radiation is directed at the object from a single addressable point at the x-ray source for a clearly defined period of time. The x-ray source and the detector assembly are on opposite sides of the support member. For each area-wide pulse of radiation that is projected onto the object at various angles, a data sample of sub-images of non-overlapping regions is captured. Moreover, by manipulating the relative position of the object with respect to the imaging radiation projected from the source and by illuminating the object with pulses of radiation at selected intervals, a time series of successive sub-images corresponding to overlapping regions of the object are captured by each detecting module. Variations of existing mathematical algorithms reconstruct the captured sub-images to form a composite tomographic or tomosynthetic image. In one application, the sampled object is a printed circuit board (PCB).
The detector assembly also includes a supporting structure for the placement of the sparsely distributed detecting modules. The modules may be identical, but each module is coupled to a dedicated readout channel for data transmissions that are electrically isolated from each neighboring module. Thus, if a module becomes non-functional, the sparse configuration of modules enables part replacement to be limited to the defective module, rather than the entire detecting array.
In the sparse configuration, each module is strategically located on the supporting structure and is spaced apart by an intermediate distance from a neighboring module. The distance between each neighboring module may be determined on the basis of various factors, such as the specific application, economics (e.g., cost savings), and the desired throughput rate. The distance between modules may be one-quarter of the lateral dimensions of the modules, but is preferably at least as great as this lateral dimension.
Each detecting module includes a two-dimensional array of sensor elements for sensing the intensity of the imaging radiation emerging from a sampled region. In one embodiment, the sensor elements are complementary metal oxide semiconductor (CMOS) pixel sensors coupled to x-ray scintillating materials. Alternatively, the sensor elements are charged coupled devices (CCDs). In yet another embodiment, utilizing a solid-state material that converts x-ray photons directly to electron-hole pairs (e.g., CdTe, CdZnTe) and directly coupling this material to a CMOS readout array eliminates the need to provide a scintillator for converting the radiation to visible light prior to detection.
An x-ray source may include a source tube and optics for projecting pulses of imaging radiation from a continuous region onto the sampled object. The continuous region may be a xe2x80x9cspot,xe2x80x9d which is herein defined as an area on the anode (or illuminating surface) of generally circular shape from which the imaging radiation is projected in a broad range of angles. The projected penetrating radiation causes each sub-image of a region of the sampled object to be captured at a unique projecting angle. The projecting angle is defined as the angle of incidence measured between the normal of the module and the sampled radiation that is projected onto the module.
For each pulse of imaging radiation that is projected, sub images of multiple non-overlapping regions are captured by the modules, with each module receiving imaging radiation from a range of projected angles.
Accordingly, each capture by a respective module may subsequently be used in forming an image of the object.
In accordance with the invention, the x-ray source and the sampled object are configured to achieve relative displacements between the imaging radiation and the sampled object. These displacements may occur during the time between successive pulses. Preferably, manipulation of the x-ray spot provides the primary displacement for the collection of data sets of the targeted region. For example, the source may be physically moved or the spot within the source is moved. The x-ray spot is formed by bringing a high energy finely focused beam of electrons (30 to 250 kV or greater) to strike a target typically fabricated from an efficient x-ray fluorescing material with suitable melting point and stopping power (e.g., Cu, Mo, W). Both by natural fluorescence and brehmstrahlung processes, a broad spectrum of x-ray photons is emitted over a wide range of angles. The position of the spot can arbitrarily be set simply by positioning the beam of electrons within the sourcexe2x80x94a procedure very similar to the operation of a scanning electron microscope.
The relative displacement between the projected x-ray radiation from the source and the object may be provided by linearly moving the support member in one direction (e.g., x direction) and the x-ray spot position in a perpendicular direction (e.g., y direction). Alternatively, the x-ray source position can be moved. In other embodiments, relative displacements include moving the x-ray spot position or the support member in both the x and y directions. The x-ray spot position and/or the support member can be configured to move in incremental steps or at uniform velocity. The generation of the pulses should be frequent (i.e., more than one pulse generated per object region), so that the time series of sub-images captured by each detecting module is of overlapping regions of the object. An increase in the sparsity of the detecting modules (e.g., the intermediate distances between modules are at least as great as the side-to-side distance across a module) necessitates an increase in the number of pulses per region examined, if a desired result quality for the reconstructed image is to be maintained. That is, there is a tradeoff between reducing manufacturing costs by decreasing the module density for a given-size detector assembly and the throughput enabled rate to maintain a given image rendering.
For each sub-image captured by each detecting module, parameter data must be identified. The parameter data includes position data that is indicative of the position of the xe2x80x9ccurrentxe2x80x9d pulse relative to the sampled object at the time that the sub-image was captured and includes angular data that indicates the projection angle for the sub-image. Moreover, timing data corresponding to the time interval between pulses during relative displacement at constant velocity may be identified. The sub-images and the corresponding parameter data are transmitted to an integrator unit. Depending on the type of computational algorithm used for reconstruction, different sets of parameter data are used for reconstructing the sub-images into a composite three-dimensional image (sometimes referred to as a xe2x80x9cdescriptionxe2x80x9d) of the object or a two-dimensional slice from the three-dimensional image.
One of the advantages of the sparse configuration is that manufacturing costs are reduced as a result of the reduction in the total number of sensor elements, thereby allowing an arbitrary large tomographic angle to be obtained without the scaling costs associated with a large single detector. Additionally, the invention provides the ability to replace a single defective detecting module if it becomes non-functional, rather than replacing the entire detecting array. While the described embodiments are shown as having a number of advantages, other embodiments may not share the same advantages.